Atterberg Limits

Lab 3 – Atterberg Limits

Introduction The liquid limit and plastic limit (known as the Atterberg limits) serve as the boundaries between different soil consistencies. The plastic limit is the boundary between the semisolid and plastic states and the liquid limit is the boundary between the plastic and liquid states. When the Atterberg limits are used in conjunction with a sieve analysis, such as was performed in Lab 2, a soil can be fully classified in either the Unified Soil Classification System or the AASHTO classification system. The procedure outlined here for the determination of the Atterberg limits follows ASTM D 4318 – Liquid Limit, Plastic Limit, and Plasticity Index of Soils [1]. For the full classification of soils using information from both Lab 2 and Lab 3, ASTM D 2487 – Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System) is followed [2]. Materials and Equipment The following are required for this procedure: Plastic Limit Test  Soil sample in beaker  Spatula  Plastic squeeze bottle with water  Two moisture cans  Balance sensitive to 0.01g  Oven  Ground glass plate  Copy paper Liquid Limit Test  Soil sample in beaker  Spatula  Plastic squeeze bottle with water  Three moisture cans  Balance sensitive to 0.01g  Oven  Casagrande liquid limit device  Grooving tool Procedure Plastic Limit Test 1. Prior to lab, a 150 – 200 g soil sample was prepared and placed in the beaker. 2. Add ~20g to 25g of water to the sample from the squeeze bottle and mix thoroughly with the spatula. 3. Measure and record the mass and number of the two moisture cans. 4. Place the copy paper on the table (or on the ground glass plate) 5. Take a small portion of the moist sample and roll into a ball. 6. Roll the ball on the paper under the palm of your hand. a. Your goal is to form small threads of soil (see Figure 1) that begin to crumble when their diameter is ~1/8”. 7. Once the thread is formed, place it in the moisture can. Keep adding threads until each moisture can has ~13g of sample. 8. Measure the mass of each moisture can with the sample. 9. Place each moisture can in the oven to dry for 24 hours. 10. Return after 24 hours to measure and record the mass of each can plus the dry sample. Figure 1: Soil threads that are ~1/8” diameter Liquid Limit Test **The procedure for the liquid limit test will be repeated three times. Ideally, the sample will be the driest for the first test, and you will add water (i.e. increase the moisture content) for the second and third tests. Adding water should decrease the number of blows required to achieve groove closure. The ideal number of blows per test is: First test: 25 ≤ ? ≤ 35 Second test: 20 ≤ ? ≤ 25 Third test: 15 ≤ ? ≤ 20 1. Measure and record the mass and number of three moisture cans. 2. Take the sample that was used in the plastic limit test, add a small amount of water and mix thoroughly. CAUTION: if too much water is added at the beginning of the test, the number of blows required to close the groove will be too low (<25) and the sample will need to be stirred with the spatula to allow it to air dry. 3. After the sample has been mixed thoroughly, place a portion of it in the liquid limit device. Use the spatula to smooth the surface of the sample as seen in Figure 2. Figure 2: Sample in liquid limit device prior to testing. 4. Cut a groove in the center of the sample using the grooving tool as shown in Figure 3. Figure 3: Groove in soil sample. 5. Turn the crank of the LL device at a rate of ~2 rev/s. Count the number of blows required for ½” of the groove to close. See Figure 4. Figure 4: Sample after grove has closed. 6. For the first test, if N is between 25 to 25 blows, collect a portion of the sample with the spatula and place in the moisture can. See Figure 5. Determine the mass of the can plus the mass of the sample. Figure 5: LL device before and after sample collection. 7. Clean the remaining sample from the LL device and clean the device carefully with paper towels. 8. Add more water to the sample and mix thoroughly. 9. Repeat steps 3 – 8 twice. The second test should achieve groove closure between 20 – 25 blows and the third test should achieve groove closure between 15 – 20 blows. 10. Place the three moisture cans in the oven to dry for ~24 hours. Measure the mass of the moisture can plus the sample after it has been oven-dried. Lab Report Requirements 1. Complete the calculations in Table 3.1 on the next page. Calculate the plastic limit by taking the average of the two plastic limit tests. 2. Plot the moisture content vs. the number of blows for the liquid limit test results. Use a semi-log plot. Include the flow curve on the graph. Determine the liquid limit of the soil by finding the moisture content that corresponds to 25 blows. 3. Use the calculated plastic and liquid limits to determine the plasticity index. 4. Fully classify the soil using the data from both Lab 2 and Lab 3 using the Unified Soil Classification System (USGS). See the figures on the next several pages. a. Provide an outline of the steps that lead to the final classification. b. Give the Group Symbol and Group Name. c. Comment on the location of the soil data point in the Plasticity Chart. 5. Estimate the shrinkage limit of the soil using the plasticity chart. Table 3.1 – Plastic and Liquid Limit Data Table Soil Sample Description: Plastic Limit Liquid Limit Group #: ____ Test 1 Test 2 Test 3 Test 4 Test 5 Can/Lid Number Can/Lid Mass, Mc (g) Can/Lid + Moist Specimen Mass, Mcms (g) Date and Time In Oven Can/Lid + Oven Dry Specimen Mass, Mcds (g) Date and Time Out of Oven Mass of Water, Mw = Mcms – Mcds (g) Mass of Soilds, Ms = Mcds – Mc (g) Water Content, w = (Mw/Ms) * 100% (%) Number of Blows Liquid Limit = ___________ Plastic Limit = ___________ Plasticity Index = LL – PL = _________ Figure 6 – USCS Classification Table Figure 7 – Plasticity Chart Figure 8 – Flow Chart for Group Names and Symbols

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